Adipose tissue B cells are the second most abundant immune cells found in adipose tissue, and they account for more than 20% of the stromal population within adipose tissue during obesity. Pre-clinical and clinical studies have only recently suggested that adipose tissue B cells can modulate adipose tissue function and contribute to the metabolic syndromes suffered during obesity. However, our understanding of the molecular mechanisms underlying adipose tissue B cell function is limited, and this has significantly hampered the potential translational benefit of targeting B cells that dwell in adipose tissue to mitigate obesity induced metabolic disease. The central goal of our project is to define the key regulators for these B cells during obesity. Nevertheless, our new data has opened a novel point of intervention to control the function of these inflammation-promoting B cells through the action of microRNAs that include miR-150. Our data support the central hypothesis suggesting that: Under the stress of obesity, miR-150 is a key regulator of adipose tissue B cell activation where dysregulation of miR-150 facilitates the pro-inflammatory response of adipose tissue B cells and corrupts their ability to interact with other cells in the adipose tissue niche, which results in exacerbated adipose tissue inflammation and insulin resistance. We will critically test this hypothesis in three specific aims by first demonstrating how miR-150 regulates normal B cell function. This will be achieved with the creation of new and unpublished mouse strains that will allow exquisite control of miR-150 action in B cells during different stages of development. Specifically, we will utilize our novel miR-150 B-cell specific knockout and overexpression mice to define how adipose tissue B cells function under lean and obese conditions. In the second aim we will determine the regulatory role of the gene targets of miR-150 in controlling adipose tissue B cell behavior and function. Specifically, miR-150 regulates B cell pathways through specific target genes and we have screened more than 30 predicted genes and successfully identified several B cell specific miR-150 targets. We will utilize ectopic expression and shRNA knockdown to determine their roles in miR-150-mediated ATB activation and subsequent in vivo function in obese adipose tissue. In aim 3 we will test the hypothesis that disruption of miR-150 in adipose tissue B cells interferes with their ability to control cell-to-cell interactions. We will determine the mechanistic regulation of miR-150 during crosstalk with other cell populations including T cells and macrophages derived from adipose tissue and also adipocytes themselves. We have postulated that part of the mechanism used by the pro-inflammatory B cells is through the MHC II pathway, which will be tested under various physiological conditions. Collectively, these three aims will elucidate the critical regulatory mechanism of microRNA-regulated adipose tissue B cells and their contribution in orchestrating the adipose tissue metabolic/immunologic response to obesity.